FIG. 7 shows a typical electronic flash unit basically comprising: flashlamp 20 for instance a xenon lamp (set inside of a reflector, but not shown); lamp unit 4 provided with trigger terminal 21; trigger circuit 10 provided with trigger coil 11 and trigger capacitor 13; transformer 14 composed of transformer coil 15; accumulator 16 composed of main capacitor 17; and power source 22 composed of DC source 23. Main capacitor 17, on a charged condition of for instance 300 V by DC source 23, allows trigger circuit 10 to apply a several kV of trigger voltage on trigger terminal 21 of flashlamp 20, thereby causing flashlamp 20 to ignite for flashing by discharging the power storage in main capacitor 17.
Other types of electronic flash units would have dimming circuits 25 provided with Insulated Gate Bipolar Transistor (IGBT) 26, optical sensor 27 or the like added to the aforesaid configuration to control the amount of flashing light by dimming circuits 25.
A lot of conventional electronic flash units like described above are used in various kinds of cameras such as silver film cameras, well known digital cameras employing CCD or CMOS sensor as an imaging element, and single-use-cameras having a simplified structure of a film incorporated with a lens unit and the electronic flash unit built-in the camera body.
As portable telecommunication terminals or cell-phones capable of transmitting stored images using built-in digital camera have become popular in the market, electronic flash units detachable to the cell-phones have also begun to be provided. Along with recent trends of downsizing in cameras and cell-phones, or built-in design in cell-phones, a more compact sized electronic flash units are required to meet the market needs.
Downsizing of the electronic flash unit, however, cannot be achieved without solving the problem of so-called “trigger leak”. The trigger leak means a poor flashing due to occurring a creeping discharge that results in an ineffective ionization in flashlamp 20 caused by a decrease in clearance between trigger terminal 21 provided on the glass bulb of flashlamp 20 and main electrode 20a of flashlamp 20.